1. Field of the Invention
This invention relates generally to fabrication of a semiconductor device and more particularly to a method for forming an L-shaped spacer using a sacrificial organic top coating.
2. Description of the Prior Art
As the physical geometry of semiconductors continues to shrink, the space between gate electrodes shrinks as well. The reduced space between gate electrodes can cause poor gap fill by the subsequent dielectric layer (eg inter-layer dielectric) resulting in degraded performance.
One approach which has been used to solve this problem is the use of spacers on the gate electrode sidewalls which are smaller at the top than they are at the bottom, such as L-shaped spacers. In a typical L-shaped spacer two dielectric layers (the first composed of silicon nitride and the second composed of silicon oxide) are formed over and around a gate electrode, then anisotropically etched. However, the top oxide portion of the spacer can not be easily removed without damaging oxide isolation structures. Conversely, if the top oxide portion of the spacer remains, it can be effected by post-etch wet chemical process causing inconsistent spacer shape and size across the IC and between IC's. Also, the gap filling ability of the subsequent dielectric layer is not optimized, because the aspect ratio remains high due to the presence of the oxide portion of the spacers.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 4,838,991 (Cote et al.) recites a process for defining organic sidewall structures on a photoresist pattern to enhance a photolithography process.
U.S. Pat. No. 5,858,847 (Zhou et al.) shows a LDD process using a photoresist block over a hard mask and surrounded by a polymer layer to provide a highly-doped source and drain implant mask, then removing the polymer layer and photoresist block for the lightly-doped source and drain implant.
U.S. Pat. No. 4,868,617 (Chiao et al.) and U.S. Pat. No. 5,024,959 (fiester) show L-shaped spacers and processes.
U.S. Pat. No. 5,882,973 (Gardner et al.) shows double spacers used to increase the length of source and drain extensions for particular transistors in an IC.
U.S. Pat. No. 5,234,852 (Liou) shows sloped spacers formed using a reflowable material.
U.S. Pat. No. 5,770,508 (Yeh et al.) shows a process for forming an L-shaped spacer using an oxide layer and an overlying nitride layer, anisotropically etching to form spacers, then removing the nitride layer, leaving an L-shaped oxide spacer. Ions are implanted through the L-shaped spacer to form shallow source and drain extensions under the L-shaped spacer and moderate depth source and drain regions beyond the L-shaped spacer.